Jim Chanos and Carson Block, two of Wall Street's biggest short sellers, fired shots at Sunrun , saying the residential solar company is not as "green" as many investors think. "This company is the aluminum siding of the 21st century for those who are old enough to know what I'm talking about," Chanos, founder of Kynikos Associates, said at CNBC's Delivering Alpha Investor Summit in New York City Wednesday. Aluminum re-sidings were prevalent in the 70s and 80s but they fell out of favor as the material was susceptible to scratches and dents. "It's a roofing company. It's basically a roofing company with a second mortgage subsidiary attached to it. It's not a green company. It doesn't produce solar panels," Chanos added. Chanos said Sunrun doesn't produce solar converters. Instead, it buys converters from other and leases to its customers, Chanos said. Muddy Waters Capital CIO Carson Block said he has a short position against Sunrun, taking issue with the way the company deals with tax subsidies from the government. "To the extent that the subsidies increase, this just enables companies like Sunrun to destroy more capital or more value in a shorter period of time," Block said at the conference. Block said he suspects that Sunrun classifies a lot of its sales and marketing expenses as other operating expenses. "They have tax issues and a myriad of other things," Chanos said. "But because it's seen as ESG because it's perceived, that's the variant perception and that's where you can make a lot of money on the short side when everybody believes this." Shares of Sunrun are down about 9% this year. Last month, Sunrun responded to Muddy Waters' attack, saying the short seller tried to mislead their readers with "grossly false and inaccurate assertions in a brazenly self-serving effort" to mitigate recent losses in their short position. "Muddy Waters has its 'facts' wrong. For over 10 years, our investors, lenders and independent authorities have closely diligenced our tax and valuation procedures, which Muddy Waters incorrectly describes," said Mary Powell, Sunrun's CEO and Director, in a statement. "Sunrun works hard to educate all of our stakeholders, and we appreciate that our investors have spent the time to understand renewable energy financing structures."